FAQ - Sealing a conflat flange and flanges in multiples

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Richard Hull
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Re: FAQ - Sealing a conflat flange and flanges in multiples

Post by Richard Hull » Thu Dec 05, 2019 11:37 pm

Dan is correct. Crushing the copper gasket is the only way to seal as the knife edges crush into the copper. Tighten in a pattern as I noted in my original FAQ and in a manner given in great detail by Dan. Pay attention to the vacuum gauge and the gap between faces as you tighten. Hopefully, you will seal up well before the faces meet. If you have to tighten to where the faces meet and still can't have the pressure drop, you have a really bad knife edge and a bad flange. A flange damaged this badly is easy to see with the eye at a glance. Sometimes a 2.75 elastomer gasket will work with a badly damaged knife edge, but you have partially defeated the reason for the CF flanges altogether...(very high vacuum). If the elastomer gasket doesn't seal, throw this flange in the scrap stainless steel bin.

I bought a pack of 5 elastomer 2.75 gaskets years ago, but never had an occasion to use them as all my copper gaskets worked well. They are a last resort only.

To answer more direct you query regarding torquing on the bolts. Yes, proceed to infinite torque if you can't seal well, if you wish, and feel free to break off bolt heads if you are man enough. In general, if it doesn't seal well with 1000 pound-feet of torque, once the bolt head snaps off, it will never ever seal. There is no torque rating specified in the vacuum world for CF flange sealing. You are not putting on the heads in a gasoline engine here. You just want a vacuum tight seal. What actually gets you there is the precise amount and good enough.

Richard Hull
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